The car has a 4K rev limiter for P/N to protect the torque converter from operating at high RPM w/no load. It works by dropping a couple of cylinders. It is over-ridden by a speed senor when the car is moving.
Next time it starts to misfire on the road, put it in N and see if it clears up.
Should go up on tach easily.
That will help narrow down the diagnosis and indicate that part of the system needs to be checked.